Apparatuses for large area radiation detection and related method

ABSTRACT

Apparatuses and a related method relating to radiation detection are disclosed. In one embodiment, an apparatus includes a first scintillator and a second scintillator adjacent to the first scintillator, with each of the first scintillator and second scintillator being structured to generate a light pulse responsive to interacting with incident radiation. The first scintillator is further structured to experience full energy deposition of a first low-energy radiation, and permit a second higher-energy radiation to pass therethrough and interact with the second scintillator. The apparatus further includes a plurality of light-to-electrical converters operably coupled to the second scintillator and configured to convert light pulses generated by the first scintillator and the second scintillator into electrical signals. The first scintillator and the second scintillator exhibit at least one mutually different characteristic for an electronic system to determine whether a given light pulse is generated by the first scintillator or the second scintillator.

GOVERNMENT RIGHTS

This invention was made with government support under Contract NumberDE-AC07-051D14517 awarded by the United States Department of Energy. Thegovernment has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.12/683,904, entitled “Method, Apparatus and System for Low-Energy BetaParticle Detection,” filed Jan. 7, 2010, now U.S. Pat. No. 8,274,056,issued Sep. 25, 2012, the disclosure of which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

Embodiments of the present disclosure relate generally to materialdetection and characterization of an environment and, more specifically,to a method, apparatus and system for detection and characterization ofmaterials emitting low-energy beta particles.

BACKGROUND

Low-energy beta particle-emitting radionuclides, such as Technetium-99(Tc-99), can be difficult to measure in the environment in which theyare located. Tc-99 is one of the fission products with a relatively highyield in the thermal neutron fission of Uranium-235 (U-235) orPlutonium-239 (Pu-239), and Tc-99 has a long half-life of approximately2.1×10⁵ years. Tc-99 is found in the environment due to fallout frompast atmospheric nuclear weapons tests and reactor fuel processing. Inaddition, Tc-99 is mobile in the environment, can move into groundwater,and may constitute a health hazard to humans if taken into the body. Forthese reasons, it may be desirable to be able to detect the presence ofTc-99. Conventional methods for detecting Tc-99 in soil include samplingthe soil and running chemical tests on the soil to determine Tc-99concentration.

Many conventional radiation detectors are designed to detecthigher-energy emitting radionuclides, especially those radionuclidesthat emit gamma rays that travel a great distance due to the high energyof gamma rays. The only emissions that come from Tc-99 are low-energybeta particles, which absorb quickly and do not travel very far in airor soil. As a result, detectors designed to measure radioactive activityremotely, from a significant distance, may not be useful for detectionof such low-energy beta particle-emitting radionuclides.

Further, conventional radiation detectors are not designed todiscriminate between higher-energy radiation emitters and low-energybeta emitters, which may be either man-made or natural (e.g., K-40). Notbeing able to discriminate between these emitters makes it difficult toknow what contribution of the radiation measured is attributable toTc-99. Part of the difficulty in using conventional radiation detectorsto detect such low-energy emitting radionuclides is due to the expectedpresence of a number of other radionuclides in the environment beingexamined, which presence may cause background interference. Thebackground interference may be caused by high-energy gamma- orbeta-radiation emitters interacting with the detector. Table 1, below,lists the nuclear properties of Tc-99 along with the expectedcontaminants and concentrations in an environment (e.g., the vadosezone) to be examined.

TABLE 1 Beta Endpoint Gamma ray Expected Radio- Radiation and EnergyEnergies Concentration nuclide yield (%) (keV) (keV) (pCi/g) Tc-99β-100% 292 Low yield    1-14,000 gamma Cs-137 β--94% 511 661 1  (6%)K-40 β--89% plus low 1300 1461 10-40 yield positron emitter (11%) U-235A 4-4.5 MeV 142.8, 185 1-3 β- -daughter products (46%) U-238 A-415 to420 MeV 1000 from 1-3 β- -daughter products daughter Th-232 A 3.8-4.0MeV Multiple 1-3 β- -daughter products gamma rays Co-60 β-100% low 3181173 and  2-20 yield at 1500 1332

As indicated by Table 1, a number of other contaminant radionuclides areexpected to be present in the environment where Tc-99 may likely bepresent. These other radionuclides may include Cesium-137 (Cs-137),Potassium-40 (K-40), Uranium-235 (U-235), Uranium-238 (U-238),Thorium-232 (Th-232), and Cobalt-60 (Co-60). Other expected contaminantradionuclides not listed in Table 1, but discussed later, includeStrontium-90 (Sr-90), Yttrium-90 (Y-90), and Tritium (H-3).

Referring again to Table 1, Tc-99 exhibits an essentially 100% yield ofbeta-particle emission with little to no gamma ray emission. The betaendpoint energy is the maximum energy beta particles when the betaparticles exit the nucleus before they scatter and lose some energy inthe environment. As shown on Table 1, Tc-99 has the lowest beta particleendpoint energy of the radionuclides expected to be present in thetarget environment. The other radionuclides listed have various yieldsand energies of beta particle and gamma ray emissions. With thelow-energy beta particle emission, the quick absorption of betaparticles, and the existence of other higher-energy emittingradionuclides (both beta particles and gamma rays) in the environmentcausing background interference, detection of low-energy betaparticle-emitting radionuclides, such as Tc-99, may prove difficult.This is especially true as Tc-99 is only a beta particle emitter, and nogamma rays, with their accompanying deeper penetration, are emitted.

Some conventional phoswich detectors have been used to discriminatebetween different types of radiation to determine which types ofradiation are present (e.g., discriminate between alpha, beta, and gammaradiation); however, such detectors are not configured to discriminatebetween different sources of the same radiation type (e.g., betweendifferent beta emitters). The inventors have appreciated that there is aneed to discriminate between sources of the same radiation type,including low-energy beta emitters such as Tc-99, and to do so in theenvironment where the radiation source exists without the need to take amaterial sample to run chemical analyses off-site in a laboratory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded schematic view of a stacked configuration of adetector assembly according to an embodiment of the present disclosure;

FIG. 2 is a schematic of a concentric configuration of a detectorassembly according to an embodiment of the present disclosure;

FIG. 3 is a schematic of a detector assembly with a third scintillatorand a fourth scintillator according to an embodiment of the presentdisclosure;

FIG. 4 is a hardware/software block diagram of a radiation detection anddiscrimination system for measuring a low-energy beta particle emitterin the presence of other beta and gamma radiation emitters according toan embodiment of the present disclosure;

FIGS. 5A and 5B are plots of the time/energy spectrum of Tc-99 asmeasured by a detector assembly and as generated by a spectrometeraccording to an embodiment of the present disclosure;

FIGS. 6A and 6B are plots of the time/energy spectrum of Sr-90 asmeasured by a detector assembly and as generated by a spectrometeraccording to an embodiment of the present disclosure;

FIGS. 7A and 7B are plots of the time/energy spectrum of Cs-137 asmeasured by a detector assembly and as generated by a spectrometeraccording to an embodiment of the present disclosure;

FIG. 8 is a schematic diagram of a large area radiation detector systemaccording to an embodiment of the present disclosure;

FIG. 9 is a schematic diagram of a large area radiation detector systemaccording to another embodiment of the present disclosure; and

FIG. 10 is a schematic diagram of a large area radiation detector systemaccording to another embodiment of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof and, in which is shown byway of illustration, specific embodiments in which the disclosure may bepracticed. These embodiments are described in sufficient detail toenable those of ordinary skill in the art to practice that described inthis disclosure, and it is to be understood that other embodiments maybe utilized, and that structural, logical, and electrical changes may bemade within the scope of the disclosure.

In this description, functions may be shown in block diagram form inorder not to obscure the present disclosure in unnecessary detail.Furthermore, specific implementations shown and described are onlyexamples and should not be construed as the only way to implement theembodiments of the present disclosure unless specified otherwise herein.It will be readily apparent to one of ordinary skill in the art that thevarious embodiments of the present disclosure may be practiced bynumerous other partitioning solutions. For the most part, detailsconcerning timing considerations, and the like, have been omitted wheresuch details are not necessary to obtain a complete understanding of thepresent disclosure in its various embodiments and are within theabilities of persons of ordinary skill in the relevant art.

When executed as firmware or software, the instructions for performingthe methods and processes described herein may be stored on acomputer-readable medium. A computer-readable medium includes, but isnot limited to, magnetic and optical storage devices such as diskdrives, magnetic tape, CDs (compact discs), DVDs (digital versatilediscs or digital video discs), and semiconductor devices such as RAM,DRAM, ROM, EPROM, and Flash memory.

Referring in general to the following description and accompanyingdrawings, various embodiments of the present disclosure are illustratedto show its structure and method of operation. Common elements of theillustrated embodiments are designated with like reference numerals. Itshould be understood that the figures presented are not meant to beillustrative of actual views of any particular portion of the actualstructure or method, but are merely idealized representations employedto more clearly and fully depict the present disclosure defined by theclaims below.

It should be understood that any reference to an element herein using adesignation such as “first,” “second,” and so forth does not limit thequantity or order of those elements, unless such limitation isexplicitly stated. Rather, these designations may be used herein as aconvenient method of distinguishing between two or more elements orinstances of an element. Thus, a reference to first and second elementsdoes not mean that only two elements may be employed or that the firstelement must precede the second element in some manner In addition,unless stated otherwise, a set of elements may comprise one or moreelements.

Embodiments of the present disclosure may include detecting specificlow-energy radiation emissions and discriminating these emissions in thepresence of other beta and gamma radiation emitters. One example of sucha low-energy beta particle emitter is Tc-99. Because of Tc-99'slow-energy beta particle emissions, and a radioactive yield ofessentially 100% beta particle emissions, Tc-99 may be well-suited fordetection with the apparatuses, methods, and systems described herein.However, embodiments of the disclosure may be applicable to other betaparticle emitters. However, for ease of description, the non-limitingexamples discussed herein will refer to Tc-99 as the beta emitter thatis desired to be detected, and for which certain configurations and/ordesign decisions may be illustrated.

FIG. 1 is an exploded schematic view of a stacked configuration, whichmay also be characterized as an axial detection configuration, of adetector assembly 100 according to an embodiment of the presentdisclosure. Detector assembly 100 may include a first detector in theform of first scintillator 110, a second detector in the form of secondscintillator 120, a light-to-electrical converter 130, and a guardelement 140. The first scintillator 110 and the second scintillator 120may be located adjacent one another such that at least a portion oflight pulses generated within the first scintillator 110 filters intothe second scintillator 120 and reaches the light-to-electricalconverter 130. The light-to-electrical converter 130 is operably coupledto the second scintillator 120 in order to receive the light pulsesgenerated within the first scintillator 110 or the second scintillator120, and convert the light pulses into electrical signals for furtheranalysis and/or storage by an electronic system. Light-to-electricalconverter 130 may be a photomultiplier tube (PMT), photodiode,charge-coupled device (CCD), complementary metal-oxide semiconductor(CMOS) image sensor, or other suitable device for converting receivedlight-to-electrical signals.

With the physically adjacent locations of the first scintillator 110 andthe second scintillator 120, as well as with the transition to thelight-to-electrical converter 130, the light pulses may experience somesignal loss before the light pulses generated by the first scintillator110 and the second scintillator 120 reach the light-to-electricalconverter 130. One way to increase coupling (i.e., reduce coupling loss)at these transitions may be to use optical coupling grease, to bond thematerials, or through other methods known in the art to better couplethe light transmission from the surfaces between the first and secondscintillators 110, 120. Signal losses in the light pulses may also beexperienced inside individual scintillators 110, 120 themselves. Outersurfaces of the first and second scintillators 110, 120 may be prepared(e.g., polished or reflectively coated) in order to reduce the amount oflight scatter out the sides of the scintillators 110, 120. Reducinglosses from scattering may result in less uncertainty in detection, asmore light pulses may be contained and received by thelight-to-electrical converter 130.

A guard element 140 may also be coupled with an outer surface of thefirst scintillator 110, and be configured to protect the surface of thefirst scintillator 110. The guard element 140 may be further configuredto act as a shield to prevent certain alpha and very low energy betaparticles (e.g., H-3) from reaching the first scintillator 110 but toallow beta particles from Tc-99, and other higher-energy radiation toenter the first scintillator 110. Such alpha and beta particles may havea lower energy than the energy from the low-energy beta particle emitter(e.g., Tc-99) to be detected. It may be desirable for the detectorassembly 100 to measure these lower-energy radiations. In thatsituation, a third scintillator (see FIG. 3) may be adjacent to theouter surface of the first scintillator 110 between (or in place of) theguard element 140 to measure and discriminate alpha particle emittersand low-energy beta emitters (e.g., H-3) from other low-energy betaemitters (e.g., Tc-99).

In operation, a radiation source 50 emits radiation 55, which isreceived by detector assembly 100. The radiation source 50 may be soil,air, or water, which substance is contaminated with at least one of anumber of radionuclides including, but not limited to, Tc-99, H-3,Sr-90, Y-90, Cs-137, Co-60, Th-232, K-40, U-235, and U-238. Theseradionuclides may also be referred to herein for convenience asradiation sources 50. Such radiation sources 50 may emit radiation 55including alpha particles, beta particles, and gamma rays. Whenradiation 55 passes through some material, there is some probabilitythat the radiation 55 will interact with the material and lose someenergy. Low-energy radiation has a high probability of interaction atshort distances, and higher-energy radiation will have some lowerprobability for interactions at short distances from the radiationsource 50.

Radiation 55 interacts with the first and second scintillators 110, 120,which generate light pulses responsive to low-energy beta particles,high-energy beta particles, and gamma rays. These light pulses arereceived by the light-to-electrical converter 130 for conversion toelectrons that may be processed by electronic hardware and software (seeFIG. 4) for analysis and storage. The first and second scintillators110, 120 exhibit at least one mutually different, distinguishingcharacteristic in the detector assembly 100 response such that theelectronic hardware and software can determine which light pulses weregenerated in the first scintillator 110 and which light pulses weregenerated in the second scintillator 120 and the energy of those lightpulses. An example of such a distinguishing characteristic is the pulseshape characteristic (i.e., rise time) for the light pulses respectivelygenerated in the crystals of the first and second scintillators 110,120. The rise time is a characteristic of the chemical characteristicsof the scintillator material, commonly termed the “element.”Scintillator materials may comprise crystals, plastics, or othermaterials that exhibit the property of luminescence. As between specifictypes of radiation (e.g., beta, alpha, gamma) the scintillator materialwill have the substantially same rise time regardless of the sourcestrength of radiation 55. For each particular type of radiation, each ofthe first and second scintillators 110, 120 will exhibit particularpulse shape characteristics, which the electronics may be configured torecognize. Because the electronics may also be configured to measure therise time of the light pulses, the relative pulse contribution from eachof first and second scintillators 110, 120 may be determined. Further,the energy of the pulses generated in both the first and secondscintillators 110, 120 may also be measured, thereby allowingdiscrimination to be performed both based on rise time and depositedenergy.

The energy of the beta particles emitted by Tc-99 is low compared toother radiation 55 in the environment, and the first scintillator 110 isstructured to permit substantially all of the beta particle energydeposition of the Tc-99. The first scintillator 110 is also structuredto permit other radiation 55 to be passed therethrough to the secondscintillator 120 in order to interact in the second scintillator 120.Because all, or substantially all, the Tc-99 is deposited in the firstscintillator 110, the beta particle emissions from Tc-99 may not reachinto the second scintillator 120. In other words, the first scintillator110 may be customized for the detection of a specific low-energy betaparticle source (e.g., Tc-99).

The first scintillator 110 may be structured with a thickness, for theselected material thereof, such that a majority of the beta particleenergy from the low-energy beta particle source will interact within thematerial of the first scintillator 110. This thickness may be selectedbased on the material chosen for the first scintillator 110 to ensure agoal of full energy deposition of Tc-99 in the first scintillator 110.For example, if customized to determine the amount of Tc-99 in anenvironment, this structure ensures that the vast majority, if not all,of the Tc-99 beta particle emissions are detected in the firstscintillator 110, and that only minor amounts, if any at all, of theTc-99 beta particle emissions are detected in the second scintillator120. While the first scintillator 110 may be structured to have athickness such that a vast majority of beta particle energy from Tc-99will interact within the material of the first scintillator 110, thefirst scintillator 110 thickness may not be overly thick such that theenergy of other, higher-energy beta particles is fully deposited in thefirst scintillator 110. The first scintillator 110 may, therefore, bestructured with a thickness such that the other radiation emissionsexpected in the environment (e.g., gamma rays, higher-energy betaparticles) can pass to the second scintillator 120. However, it may bedesirable for the second scintillator 120 to be of sufficient thicknessto have the energy of at least other, higher-energy beta particles befully deposited. For example, the second scintillator 120 may have athickness (e.g., 2.1 cm) for detection of higher-energy beta particles(e.g., >300 keV) and gamma rays. The selected thickness may be based onthe material chosen for the second scintillator 120 to ensure a goal offull energy deposition of higher-energy beta particles and otherexpected radiation, such as gamma rays, interacting with the secondscintillator 120. Selecting the thickness of second scintillator 120 forfull energy deposition of high-energy beta particles based on thematerial chosen for second scintillator 120 allows both rise time andenergy discrimination to be more accurately performed and quantified.For example, full energy deposition in the second scintillator 120 maypermit discrimination of high-energy beta emitters (e.g., Sr-90).

Beta particle and gamma ray interactions in the second scintillator 120may be used as anticoincidence triggers to remove higher-energy betaparticle or gamma interactions from the data obtained from the firstscintillator 110. For example, pulses in second scintillator 120 thatoccur substantially simultaneously with pulses in the first scintillator110 may be considered to have been generated by the same radiationsource 50. Because the first scintillator 110 is structured to have all,or substantially all, of the energy of Tc-99 deposited therein, thesesubstantially simultaneous pulses in the second scintillator 120 and thefirst scintillator 110 can be assumed to be from a radiation sourceother than Tc-99. As a result, the simultaneous pulses detected by boththe first scintillator 110 and the second scintillator 120 may besubtracted from the total response from the first scintillator 110,because such anticoincidence data detected in the second scintillator120 indicates that the corresponding pulses in the first scintillator110 resulted from interactions of higher-energy sources passing throughthe first scintillator 110 into the second scintillator 120. Because ofthe different penetrations of the different types of radiation 55, theunique energy characteristics of the different radiation sources 50, andmutually different characteristics of the first and second scintillators110, 120, the light pulses from the first scintillator 110 and secondscintillator 120 may be used to discriminate Tc-99 from the otherbackground radiation. For example, the pulses detected in the firstscintillator 110, which can be attributed to the gamma rays, may besubtracted out from the total response in the first scintillator 110based on the timing of the response in both scintillators in order toisolate the detection of low-energy beta particles (e.g., Tc-99). Itshould be noted that “pulses” may be referred to as an analog signalgenerated when radiation interacts with scintillators, while “counts”may be referred to as a digital representation of a pulse generatedduring processing and analysis of the measured response from a detectorassembly. However, at times, “pulses” and “counts” may be usedinterchangeably to refer generally to information generated as a resultof interactions with one or more of the scintillators.

Examples of the scintillator materials used for the first and secondscintillators 110, 120 may be selected from a plurality of plasticscintillator elements with different rise time characteristics, othercrystalline scintillator materials, and other scintillation materialswith suitable optical characteristics. Examples of plastic scintillatorelements include the BC-400 series of plastic scintillator elements,which have polyvinyl toluene as the primary material. For example, aBC-404 plastic scintillator with a relatively fast pulse rise time andBC-444G with a relatively slow rise time may be used for the first andsecond scintillators 110, 120. Other crystalline scintillators may beused for one or both of the first and second scintillators 110, 120,which may include a cesium iodide (CsI) scintillator element and aeuropium doped calcium fluoride (CaF₂:Eu) scintillator element.

In one example, a BC-404 plastic scintillator element may be used forfirst scintillator 110. The BC-404 plastic scintillator element exhibitsa relatively fast time constant of about 1.8 ns. The density of a BC-404plastic scintillator element may be approximately 1.03 g/cm³. A BC-444Gplastic scintillator element may be used for the second scintillator120. The BC-444G plastic scintillator element may exhibit asubstantially slower rise time, with an overall time constant of about285 ns. The element for the second scintillator 120 may be structuredwith a thickness for the detection of higher-energy beta particles, suchas on the order of approximately >300 keV.

Although specific examples of scintillator elements are describedherein, these are to be seen as non-limiting examples. For example, thepulse shape characteristic relationship between the first scintillator110 and the second scintillator 120 may be reversed from what was justdescribed. In other words, the first scintillator 110 may exhibit aslower rise time (e.g., BC-444G) whereas the second scintillator 120 mayexhibit a faster rise time (e.g., BC-404). For ease of description, thepair of a BC-404 scintillator and a BC-444G scintillator is used in thedescription of several non-limiting exemplary embodiments herein;however, one skilled in the art will recognize that other materials,such as those mentioned above, and combinations of materials may beused, which are responsive to beta particles and have sufficientlydifferent characteristics (e.g., rise times) from each other.Non-limiting examples of the other materials may include lithium glassscintillators, other crystalline scintillator materials with suitableoptical properties such as CsI and CaF₂:Eu, and plastic scintillatorssuch as BC-400, BC-408, BC-412, BC-416, BC-418, BC-420, BC-422, BC-444,BC-490, or other BC-400 series plastic scintillators with suitableproperties.

These other materials that may be used for at least one of thescintillators have different time constants, including BC-400 (2.4 ns),BC-408 (0.9 ns), BC-412 (3.3 ns), BC-416 (4.0 ns), BC-418 (1.4 ns),BC-420 (1.5 ns), BC-422 (1.4 ns), BC-444 (285 ns), BC-490 (2.3 ns), CsI(16 ns), CsI:Na (630 ns), CsI:T1 (1000 ns) and CaF₂:Eu (940 ns). Becauseeach of the BC-400 series of plastic scintillators has polyvinyl tolueneas the primary material, the density for each of the BC-400 series ofplastic scintillators is about 1.03 g/cm³. The CaF₂:Eu material has adensity of about 3.18 g/cm³. CsI has a density of about 4.51 g/cm³. TheBC-400 series plastic scintillator elements, the CsI scintillatorelement, and the CaF₂:Eu scintillator element are available fromSaint-Gobain Crystals of Hiram, Ohio.

One factor affecting the configuration of the detector assembly 100 isthe determination of how far radiation 55 can pass through certainmaterials for full energy deposition. Radiation 55 may not travel as farthrough dense material as through less dense material. The thickness forthe first scintillator 110 may be based on the energy deposition of thelow-energy beta particles in the material used for the firstscintillator 110. Basing thickness of the first scintillator 110 on theenergy deposition of the low-energy beta particle emitter in the chosenmaterial of the first scintillator 110 ensures that the energy from thebeta particles from the low-energy beta particle emitter to be detectedwill substantially, if not all, be deposited in the first scintillator110. The distances for full energy deposition of various radionuclidesfor different materials are indicated in Table 2. The values listed inTable 2 are derived from the NIST (National Institute of Standards andTechnology) ESTAR program used to determine stopping power and range forradiation in various materials.

TABLE 2 End Point CSDA * Full Energy Energy Range Density DepositionMaterial Radionuclide (MeV) (cm²/g) (cm³/g) (cm) Soil (SiO₂) Tc-99 0.291.00E−01 1.3 7.69E−02 K-40 1.31 7.00E−01 1.3 5.38E−01 Cs-137 0.512.14E−01 1.3 1.65E−01 Sr-90 0.55 2.42E−01 1.3 1.86E−01 Y-90 2.281.30E+00 1.3 1.00E+00 H-3 0.0186 1.00E−03 1.3 7.69E−04 Air Tc-99 0.299.50E−02 1.21E−03 7.88E+01 K-40 1.31 6.45E−01 1.21E−03 5.35E+02 Cs-1370.51 2.00E−01 1.21E−03 1.66E+02 Sr-90 0.55 2.27E−01 1.21E−03 1.88E+02Y-90 2.28 1.23E+00 1.21E−03 1.02E+03 H-3 0.0186 8.00E−04 1.21E−036.64E−01 BC-400 Tc-99 0.29 8.40E−02 1.03 8.16E−02 series K-40 1.316.00E−01 1.03 5.83E−01 Cs-137 0.51 1.80E−01 1.03 1.75E−01 Sr-90 0.552.03E−01 1.03 1.97E−01 Y-90 2.28 1.13E+00 1.03 1.10E+00 H-3 0.01867.50E−04 1.03 7.28E−04 CaF₂:Eu Tc-99 0.29 1.04E−01 3.18 3.27E−02 K-401.31 7.50E−01 3.18 2.36E−01 Cs-137 0.51 2.23E−01 3.18 7.01E−02 Sr-900.55 2.54E−01 3.18 7.99E−02 Y-90 2.28 1.33E+00 3.18 4.18E−01 H-3 0.01861.08E−03 3.18 3.40E−04 * CSDA: continuous-slowing-down approximation.

As indicated in Table 2, the beta particle endpoint energy for Tc-99 is292 keV. With this endpoint energy, effectively all beta particle energywill be deposited in about a 0.082 cm thickness of a BC-400 seriesscintillator (e.g., BC-404), measured transverse to a direction ofincoming Tc-99 beta particles. As previously stated, the firstscintillator 110 is structured to have substantially all the low-energybeta particle energy deposited, and to permit higher-energy betaparticles to pass therethrough into the second scintillator 120.Consequently, the thickness of the first scintillator 110 for a givenelement material may be selected based on the full energy deposition ofthe low-energy beta particle emitter. If the BC-404 scintillator elementis used as the first scintillator 110, and Tc-99 is to be detected, thethickness of the first scintillator 110 may be approximately 0.082 cm.As shown in Table 2, distances for full-energy deposition of betaparticles in BC-404 (from radionuclides other than Tc-99) range fromabout 0.2 cm to 1 cm. A thickness of 0.2 cm may be used as an upperlimit for the thickness of the first scintillator 110 so that higherenergy beta particles may pass through the element of the firstscintillator 110 to the second scintillator 120. Depending on thematerial chosen for the first scintillator 110, other structures (e.g.,thicknesses) may be used based on the full energy deposition of the lowenergy beta particles for that material. For example, as indicated byTable 2, the full energy deposition of Tc-99 in CaF₂:Eu is about a0.0372 cm thickness. Consequently, if the CaF₂:Eu material is used asthe first scintillator 110, and Tc-99 is to be detected, the thicknessof the first scintillator 110 may be approximately 0.0372 cm. Practicalconsiderations such as cost, availability, machinability, etc., may playa role in material selection for the different, first and secondscintillators 110, 120.

As might be expected, some beta particle activity from the otherradiation contaminants may also deposit in the first scintillator 110according to known probabilities, as higher-energy radiation 55 travelsthrough the first scintillator 110. Consequently, if other beta particleemitters are present, considerable energy deposition will be expected tooccur not only in the second scintillator 120 (e.g., BC-444G), but alsoin the first scintillator 110 (e.g., BC-404). It should be noted thatalpha particles and beta particle emissions from H-3 are very low energyin comparison to the energies of other beta particle emissions, and areunlikely to penetrate the guard element 140 (10 mg/cm²) on the firstscintillator 110.

It is recognized that there will be some standoff distance between theradiation source 50 and the surface of the first scintillator 110 ofdetector assembly 100. As indicated in Table 2, the distance for fullenergy deposition of Tc-99 in air is about 79 cm. This distance, inrelation to an approximate 1 cm or so of air distance between theradiation source 50 and the first scintillator 110 as detector assembly100 is contemplated to be used, indicates that the energy deposition inair is likely to have a minimal effect on efficiency of detectorassembly 100 and be within the degree of uncertainty expected for themeasurement.

In Table 2, SiO₂ is used to estimate energy deposition in soil, as SiO₂is a suitable substitute material available in the ESTAR program. Asindicated in Table 2, the Tc-99 full-energy beta particle depositionthickness in soil is less than 0.1 cm. The soil density (e.g., 1.3g/cm³) used for this analysis is for a loose-pack soil, which may beexpected as the detector assembly 100 is removed. The otherradionuclides have full energy deposition in soil layers ranging fromabout 0.16 cm to 1 cm, thereby providing data on the radionuclideinventories needed for calculating the detector assembly 100efficiencies and response functions used to reduce backgrounds forradionuclides other than Tc-99. These functions will be discussed later.

FIG. 2 illustrates a concentric configuration, which may also becharacterized as a circumferential detection configuration, of adetector assembly 200 according to an embodiment of the presentdisclosure. Detector assembly 200 includes a first detector in the formof first scintillator 210, a second detector in the form of secondscintillator 220, and a light-to-electrical converter 230.Light-to-electrical converter 230 may be a photomultiplier tube,photo-diode, charge-coupled device (CCD), CMOS image sensor, or anothersuitable device for converting light-to-electrical signals. Thelight-to-electrical converter 230 in this example is a photomultipliertube 232 with a voltage divider 234. Detector assembly 200 may alsoinclude a guard element 240.

Detector assembly 200 may be configured using components similar tothose employed in the detector assembly 100 in FIG. 1 to the extent thatthe first scintillator 210 and the second scintillator 220 are operablyadjacent, such that light generated in each scintillator 210, 220responsive to beta particles may be received by the light-to-electricalconverter 230. The first scintillator 210 is structured to experiencefull energy deposition of a low-energy beta particle emitter, and permithigher-energy beta particles to interact with the second scintillator220. The second scintillator 220 may be structured to experience fullenergy deposition of the higher-energy beta particles. The thicknessesselected for the elements of the first scintillator 210 and secondscintillator 220 may be based on the materials used for firstscintillator 210 and second scintillator 220 and the respectivedistances for full energy deposition of the low-energy and higher-energybeta particle depositions.

One difference between the detector assembly of FIG. 2 and that of FIG.1 is that the scintillators 210, 220 are configured in a concentricarrangement rather than a stacked arrangement. In a concentricarrangement, the second scintillator 220 may be cylindrical, and issurrounded by an annular first scintillator 210, rather than beingcoupled on one end. The detector assembly 200 may be used in a down-holeenvironment or in another environment where detection from thecircumference of the detector assembly 200 may be desirable. It shouldbe noted that, as illustrated, detector assembly 200 is configured toreceive radiation not only from about its circumference, but alsothrough a distal longitudinal end thereof. Such a concentric arrangementmay be desirable for applications in which the detector assembly 200 isto detect in a contaminated test environment with radiation entering thedetector assembly 200 from directions surrounding the detector assembly220, such as in a subterranean test environment, water test environment,air test environment, etc. The concentric arrangement may provide thedetector assembly 200 with the ability to remove the response from thesecond scintillator 220 from the first scintillator 210 regardless ofthe incoming direction of the incident radiation (not shown in FIG. 2).

The guard element 240 may extend around a side periphery of the detectorassembly 200 to cover the side surface of the first scintillator 210, aswell as around a distal longitudinal end thereof, as depicted in FIG. 2.As previously discussed, the guard element 240 may be structured toperform one or more functions. One function may be to provide protectionto the outer surface of the first scintillator 210 such as duringhandling, or as the detector assembly 200 moves up and down within acased or uncased subterranean borehole or other bore. For additionalprotection, other protective structures such as a wire grid (not shown)may be disposed about the outer surface of the detector assembly 200,over guard element 240. The guard element 240 may also be structured toshield alpha particles (e.g., from H-3) and lower-energy beta particles(e.g., lower than Tc-99) from depositing in the scintillators 210, 220,which alpha and lower-energy beta particles may otherwise add tobackground noise and cause interference with the measurement of thedesired low-energy beta particles (e.g., Tc-99). Materials withshielding properties suitable for shielding low-energy beta (e.g., H-3)and alpha particles may be suitable for guard element 240, such as thepolyester material, biaxially oriented polyethylene terephthalate(boPET), which may be available under the trade name, MYLAR®, fromduPont de Nemours of Wilmington, Del. Other materials for guard element240 may include aluminum or other thin materials that can shield thealpha or low-energy beta radiation and permit desired radiation to passthrough to the first and second scintillators 210, 220. For example, thematerial used for guard element 240 may have a nominal stopping power ofabout 10 mg/cm² in order to stop the low-energy H-3 beta and alphaparticles.

In this example, the light-to-electrical converter 230 comprises aphotomultiplier tube 232 and a voltage divider 234. The voltage divider234 applies a voltage to the different pins to get a relative potentialon the back of the photomultiplier tube 232. When radiation in the formof light generated by first and second scintillators 210, 220 isreceived by the front of the photomultiplier tube 232, a signalcomprising electrons is released when each light pulse hits a phosphorusmaterial within the photomultiplier tube 232. In order to have enoughelectrons to be useful for processing electronics, these emittedelectrons are multiplied by a series of stages in the photomultipliertube 232 to amplify the signal before processing.

In operation, a tube may be driven or otherwise placed into the groundcomprising a soil environment to be tested. Detector assembly 200 may bemoved within the subterranean tube in order to detect radiation in situ.When measurements are obtained, the subterranean tube may be removedfrom the soil in order to provide more direct access to the testenvironment being measured. When the subterranean tube is removed fromthe soil environment to be tested, detector assembly 200 is left inclose proximity to the surrounding soil with only a thin, and thusnegligible, air gap therebetween for measurements. The subterranean tubemay comprise apertures in the outer area of the subterranean tube toprovide the detector assembly 200 with more direct access to theenvironment to be measured in the absence of the subterranean tube beingremoved from the soil during testing.

Examples have described use of a subterranean tube to be driven into theground for testing of a soil environment. Alternatively, the detectorassembly 200 may be used to test other environments (e.g., water, air,etc.). In an example of a water test environment, the detector assembly200 may be placed in a tube and the detector assembly 200 may be movedthroughout the water environment. The tube may comprise apertures toallow water to flow into the tube around the detector assembly 200 inorder to have more direct access to the water test environment. Detectorassembly 200 may be used without the assistance of a tube, and may beplaced directly in the test environment. For each tested environment(e.g., soil, water, air, etc.) a plurality of detector assemblies 200may also be used in spaced relationship to obtain a more widespreadcharacterization (e.g., profile) of the test environment. The spacedrelationship between the plurality of detector assemblies 200 may bewithin a single subterranean tube, separate tubes of a plurality ofsubterranean tubes, or other placement arrangements within the testenvironment. The spaced relationship between a plurality of nearbydetector assemblies 200 may be one of a uniform or non-uniformrelationship.

The radiation source 50 (FIG. 1) surrounding detector assembly 200 maycommonly have some level of radioactive contamination. Because thedetector assemblies described herein, such as detector assembly 200, areconfigured to detect low-energy beta particle emitters and discriminatebetween such emitters in the presence of other higher-energy betaparticle and gamma radiation emitters, an accurate measurement oflow-energy beta particles may be obtained without collecting a sample ofthe radiation source 50 and running chemical analyses in a laboratory.

Detector assemblies 100, 200 may each comprise a stand-alone detectorassembly that may be configured in a stacked, or axial, configuration(e.g., FIG. 1) with a detection field at one axial end, or a concentricconfiguration (e.g., FIG. 2) for use in an environment (e.g., down-hole,air, water, etc.) with a circumferential detection field. A detectorassembly 100, 200 may be incorporated in a cone penetrometer assembly tobe pushed into the soil. A detector assembly 100, 200 may also beincorporated into a multiple detector probe assembly as one of thedetectors therein, such as is described in, for example, U.S. patentapplication Ser. No. 12/608,775, which was filed on Oct. 29, 2009, nowU.S. Pat. No. 8,878,140, issued Nov. 4, 2014, and entitled Apparatusesand Methods for Radiation Detection and Characterization Using aMultiple Detector Probe, the disclosure of which is incorporated hereinin its entirety by this reference. Implementation of an embodiment of adetector assembly of the present disclosure within a multiple detectorprobe may use data processing, output files, and printout reportssimilar to those employed with the other detectors. Some modification tothe software and interface of such a multiple detector probe may bedesirable to fully integrate the data reception and analysis of adetector assembly 100, 200 as described herein into a multiple detectorprobe.

Although the embodiments illustrated by FIGS. 1 and 2 show twoscintillators, more scintillators may be used to further discriminatebetween beta particle emitters and/or other radiation emitters. Forexample, FIG. 3 is a schematic of a detector assembly 300 with a thirdscintillator 350 and a fourth scintillator 360 in addition to the firstand second scintillators 310, 320 according to an embodiment of thepresent disclosure. In FIG. 3, third scintillator 350 may be adjacent tothe outer surface of the first scintillator 310 between (or in place of)the guard element 340 to measure and discriminate alpha particleemitters and low-energy beta emitters (e.g., H-3) from other low-energybeta emitters (e.g., Tc-99). The fourth scintillator 360 may be adjacentto the second scintillator 320, such as between the second scintillator320 and the light-to-electrical converter 330. Third and fourthscintillators 350, 360 may be in a stacked configuration such as isshown in FIG. 3, or near the center of the axis of a concentricconfiguration similar to that of FIG. 2, in order to differentiatebetween high-energy beta emitters and gamma emitters. Third and fourthscintillators 350, 360 may be configured with first and secondscintillators 310, 320 such that one or more scintillators is stacked inrelation to another scintillator and one or more scintillators isconcentric in relation to another scintillator as part of the samedetector assembly.

“Third” and “fourth” scintillators 350, 360 are used in order todistinguish from each other and from the first scintillator 310 andsecond scintillator 320. However, the existence of the fourthscintillator 360 is not dependent on the existence of the thirdscintillator 350 and vice versa. More scintillators may be used in orderto increase resolution as to the number of beta emitters and otherradiation sources to be distinguished. Each of these other scintillatorsmay be structured for full energy deposition in the material chosen fora particular radiation source to be measured, and to allow higher-energypulses to be subtracted from the total response based on, for example,anticoincidence triggers and pre-determined response functions.

FIG. 4 illustrates a hardware/software block diagram of a radiationdetection and discrimination system 400 for measuring a low-energy betaparticle emitter (e.g., Tc-99) in the presence of other beta and gammaradiation emitters according to an embodiment of the present disclosure.The discrimination system 400 may include a detector assembly 410,acquisition hardware 420, software modules 430, and a display 440.Acquisition hardware 420 may include an amplifier 425, delay 426,characteristic distinguisher 421, and spectrometer 424. As previouslydescribed, detector assembly 410 may employ at least two detectors,which may be in the form of scintillators with different characteristics(e.g., different pulse shapes such as rise times) such that theelectronic hardware and software can determine which light pulses weregenerated in the first scintillator and which light pulses weregenerated in the second scintillator. If the different characteristicfor the two scintillators includes a different rise time, thecharacteristic distinguisher 421 may include a pulse shape analyzer 422and a time-to-amplitude converter (TAC) 423. Alternatively, amulti-parameter system (not shown) may be used in place of thespectrometer 424, the pulse shape analyzer 422, the TAC 423, or anycombination thereof. The multi-parameter system may perform one or moreof the functions of the spectrometer 424, pulse shape analyzer 422 orTAC 423, including recognizing and/or quantifying the pulses based onthe rise time characteristics and energy levels of the pulses. Softwaremodules 430 may be used to perform one or more analyses on the raw datafrom the detector assembly 410 and the acquisition hardware 420 tocorrelate rise time and energy response in both the first and secondscintillators. Such analyses may include a response function analysis432, and a peak analysis 434. As non-limiting examples, amplifier 425may be an ORTEC® 460 amplifier, delay 426 may be an ORTEC® 427 delayamplifier, pulse shape analyzer 422 may be an ORTEC® 552 pulse shapeanalyzer, TAC 423 may be an ORTEC® 567 TAC single channel analyzer,spectrometer 424 may be an ORTEC® ASPEC-927 digital spectrometer, whichare available in the ORTEC® product line from Advanced MeasurementTechnology, Inc. of Oak Ridge, Tenn. An example of a multi-parametersystem is a MPA-3 series multi-parameter multi-channel analyzer systemavailable from Quantar Technology Inc. of Santa Cruz, Calif.

In operation, detector assembly 410 receives radiation 55 from aradiation source 50. The detector assembly 410 may be configured asdescribed herein with detectors in the form of a first scintillator anda second scintillator operably coupled such that light generated in eachscintillator responsive to receipt of beta particles may be received bya light-to-electrical converter (see FIGS. 1 and 2, scintillators andlight-to-electrical converter not shown in FIG. 4). The firstscintillator is structured to experience full energy deposition of alow-energy beta particle emitter, and permit a higher-energy betaparticle to pass and subsequently interact with the second scintillator.The second scintillator may be structured to experience full energydeposition of the higher-energy beta particle. The thicknesses selectedfor elements of the first scintillator and second scintillator may bebased on the materials used for the first and second scintillators andthe respective distances for full energy deposition of the low-energyand higher-energy beta particle depositions.

The interaction of the radiation 55 with the scintillators generateslight that may be received by a light-to-electrical converter. Thelight-to-electrical converter converts light to electrons that may beprocessed by the additional hardware and software modules describedherein. When light is generated by the detector assembly 410, the lightgeneration event produces information, which is separated by theacquisition hardware 420 into two separate signals for analysis. Thesetwo signals, a timing signal 427 and an energy signal 428, are receivedby the spectrometer 424 or a multi-parameter system as discussed above.The timing signal 427 indicates which of the two scintillators generateda specific light pulse. This timing discrimination is based on thedifferent rise times of the two scintillators. The energy signal 428indicates the energy of the radiation 55 at the time of the scintillatorinteraction generating the light pulse. Each radiation source 50 has acharacteristic radiation energy emission; however, according to someprobability, lower- or higher-energy emissions that are uncharacteristicto a radiation source 50 may be produced by that radiation source 50.Although some Tc-99 beta particle emissions may be determined bymeasuring a number of pulses at its beta particle endpoint energy, someof the pulses at that energy are, in all probability, generated by otherbeta and gamma radiation emitters in the environment. Thus, without acapability for compensating for these extraneous pulses caused byhigher-energy emitting radionuclides, the number of pulses for Tc-99will be too high.

As previously discussed, the two scintillators of the detector assembly410 have different signal characteristics (e.g., rise times of thesignal are different). When a light pulse is generated by the detectorassembly 410 there is a certain amount of time that elapses for a lightpulse to propagate. Depending on which scintillator generates the lightpulse, the rise-time characteristic of the light pulse is different. Bylooking at the rise times, the discrimination system 400 can determinewhich of the two scintillators contributed to that light pulse.Generally, only one or the other scintillator contributes to a givenlight pulse, but there is a possibility for a combination of bothscintillators. Depending on the radiation source, the energy generatedby the light pulse may also be different, enabling energy discriminationto be used for further analysis as described herein.

The pulse shape analyzer 422 (or a multi-parameter system) receives thesignal that comes out of the detector assembly 410 (such as, forexample, through an amplifier 425). The pulse shape analyzer 422determines the rise time of the pulse coming from the detector assembly410 and sends a start pulse and a stop pulse to the TAC 423. The startpulse is related to when the light pulse begins to generate in thedetector assembly 410. The stop pulse is related to when the light pulsestops generating in the detector assembly 410. The TAC 423 receives thestart pulse and stop pulse and calculates the time between those twoevents. The TAC 423 sends a signal to spectrometer 424, the amplitude ofthe signal being proportional to the time difference between the startand stop pulses related to the rise time to generate a given lightpulse. This proportional amplitude-to-time information allows thespectrometer 424 to create a spectrum of the different time responsesfor the different scintillators of the detector assembly 410. Thespectrometer 424 also receives energy information, which may be used tocreate a spectrum for the different energies levels received. Thespectrometer 424 may combine the time and energy spectra such that theenergy for each count can be determined for a count (i.e., light pulse),as well as in which scintillator the light pulse is generated for thatcount.

Some of the electronics used within the discrimination system 400 mayoperate on different time scales. Although these time scale differencesmay be small, unless they are reconciled, the system 400 may notfunction properly. A delay 426 may be needed so that the spectrometer424 knows which specific timing signal 427 is associated with thecorresponding energy signal 428. The delay 426 ensures that thesesignals are received by the spectrometer 424 at about the same time sothat the related timing signal 427 and the energy signal 428 areprocessed at the same time. The spectrometer 424 then sends thecorresponding time/energy information to the software modules 430 forfurther analysis. Examples of time/energy information will beillustrated in FIGS. 5A through 7B, and described herein.

Several ways are contemplated to reduce background effects in the firstdetector attributable to radionuclides other than Tc-99, and to separateout the Tc-99 pulses from extraneous pulses by higher beta and gammaemitters. One method includes using the rise-time discrimination betweenthe first scintillator and the second scintillator to obtainanticoincidence data, as described above, to subtract out of the Tc-99response of the first scintillator.

The software modules 430 receive the time/energy information from thespectrometer 424. This information may undergo a response functionanalysis 432 for radionuclides in the environment other than Tc-99,which may reduce the background effects in the first scintillator. Theresponse function analysis 432 may help determine what percentage of theobserved signal in the first scintillator is from Tc-99, and whatpercentage is from other higher-energy radiation emitters. The responsefunctions for other specific contaminant radionuclides may be employedduring the characterization of the detector to enable stripping of thespectrum of extraneous pulses in the Tc-99 response during dataanalysis. Such response functions may be determined through physicalmeasurement, mathematical simulation, or both, for each of the differentcontemplated contaminant radionuclides to determine a proportion ofpulses that generate in the first scintillator compared with the secondscintillator.

Following the anticoincidence and response function analysis,quantitative peak shape data may be measured and used with appropriatecalibration and efficiency functions to determine the quantity of Tc-99present. Peak analysis 434 includes correcting the concentration for theoverall detection efficiency of the detector assembly 410. The peakanalysis 434 may relate the detector size and the ability of thedetector to characterize Tc-99 concentrations in the environment beingmeasured with acceptable count times. Calculations may be performedbased on the expected concentrations of radionuclides in the environmentwhere the measurements are to be performed to determine the expectedefficiency and count rates for the Tc-99 detector. In this example, itis assumed that the available surface area for the Tc-99 measurement isthe outer wall of the detector assembly 410 (e.g., 6.44 cm diameter×30cm long). Another assumption is made that only beta particles from Tc-99that can reach the detector assembly 410 are within the thicknessrelated to full energy deposition in the first, or outer scintillator(e.g., approximately 0.082 cm). A third assumption is made that all betaparticles that reach the face of the detector assembly 410 are detected.Consequently, the measurable amount of Tc-99 that can be detected lieswithin 0.076 cm of the surface of the environment (see Table 2) adjacentdetector assembly 410 as the down-hole tube encompassing the detectorassembly 410 is being removed from the soil. Based on the expected lowerlimits of detection (LLD) of the detector assembly 410 with theassumptions noted above, the expected Tc-99 count rate for the detectorassembly 410 is about 350 counts/pCi for a 200 s count time. This may bean acceptable peak area to achieve appropriate counting statistics for ameasurement, particularly if the required LLD is closer to 10 pCi/g.

FIGS. 5A and 5B are plots of a time/energy spectrum 500 of Tc-99 asmeasured by a detector assembly and as generated by a spectrometeraccording to an embodiment of the present disclosure. FIG. 5A is athree-dimensional perspective plot for the time/energy spectrum 500 ofTc-99. The time axis is horizontal, the vertical axis relates toconcentration of the counts, and the energy axis is orthogonal to thepage. The rear, left-hand corner of the plot is the origin. FIG. 5B is atwo-dimensional plot of the time/energy spectrum 500 for Tc-99. The timeaxis is vertical, and the energy axis is horizontal. It should be notedthat the numbers associated with the axes of the series of plotscorrespond to channels of the spectrometer and not to any particularenergy or time values.

As shown in FIG. 5B, there are two peaks in the spectrum. One peak is atabout 400 and the other peak is at about 600 on the time axis scale.These peaks correspond to the spectra created that are proportional tothe respective rise times of the first scintillator and the secondscintillator. As shown in the three-dimensional plot of FIG. 5A, thecounts in the peak near the origin are more concentrated because of thelow energy and small energy range of the beta particles emitted byTc-99. The presence of the weaker broader peak at higher time valuesindicate that there are extra counts in the Tc-99 peak arising fromhigher-energy emissions that would be subtracted out as ananticoincidence trigger, or as part of the response function analysis.

FIGS. 6A and 6B are plots of a time/energy spectrum 600 of Sr-90 asmeasured by a detector assembly and as generated by a spectrometeraccording to an embodiment of the present disclosure. FIG. 6A is athree-dimensional plot for the time/energy spectrum 600 of Sr-90. Thetime axis is horizontal, the vertical axis relates to concentration ofthe counts and the energy axis is orthogonal to the page. The rearleft-hand corner of the plot is the origin. FIG. 6B is a two-dimensionalplot of the time/energy spectrum 600 for Sr-90. The time axis isvertical, and the energy axis is horizontal. As shown, there are twopeaks in the spectrum. One peak is at about 400 and the other peak is atabout 600 on the time axis scale. These peaks correspond to the spectracreated that are proportional to the respective rise times of the firstscintillator and the second scintillator. As shown in thethree-dimensional plot of FIG. 6A, the counts in the peak near theorigin are concentrated heavily in the first scintillator and the secondscintillator, with many more counts showing up in the secondscintillator response as shown in FIG. 6B, similar to those shown in theprevious FIG. 5B with respect to Tc-99. The high concentration of countsin both the first scintillator and the second scintillator responseindicates that the radiation energy of Sr-90 is enough to carry manybeta particles into the second scintillator; however, many interactionsstill occur in the first scintillator. During response functionanalysis, it may be determined how many counts of Sr-90 should beexpected so that those counts may be stripped from the apparent spectrumof the Tc-99.

FIGS. 7A and 7B are plots of a time/energy spectrum 700 of Cs-137 asmeasured by a detector assembly and as generated by a spectrometeraccording to an embodiment of the present disclosure. FIG. 7A is athree-dimensional plot for the time/energy spectrum 700 of Cs-137. Thetime axis is horizontal, the vertical axis relates to the concentrationof the counts and the energy axis is coming out of the page. The rearleft-hand corner of the plot is the origin. FIG. 7B illustrates atwo-dimensional plot of the time/energy spectrum 700 for Cs-137. Thetime axis is vertical, and the energy axis is horizontal. As shown,there are two peaks in the spectrum. One peak is at about 400 and theother peak is at about 600 on the time axis scale. These peakscorrespond to the spectra created that are proportional to therespective rise times of the first scintillator and the secondscintillator. As shown in the three-dimensional plot of FIG. 7A, thecounts in the peak near the origin are concentrated heavily in the firstscintillator and the second scintillator, with many more counts showingup in the response of the second scintillator as shown in FIG. 7B,similar to those shown in the previous FIGS. 5A and 5B with respect toTc-99. The high concentration of counts in both the first scintillatorand the second scintillator response indicates that the radiation energyof Cs-137 is enough to carry many beta particles into the secondscintillator; however, many interactions still occur in the firstscintillator. During response function analysis, it may be determinedhow many counts of Cs-137 should be expected so that those counts may bestripped from the apparent spectrum of the Tc-99.

FIG. 8 is a schematic diagram of a large area radiation detector system800 according to an embodiment of the present disclosure. The large arearadiation detector system 800 may also be referred to herein as a largearea beta-gamma detector system (LABS) 800. The LABS 800 includes afirst detector in the form of a first scintillator 810 and a seconddetector in the form of a second scintillator 820. The firstscintillator 810 and the second scintillator 820 may be adjacent oneanother in a stacked configuration. The LABS 800 may further include aplurality of a light-to-electrical converters 830, 831, 832, 833adjacent to the second scintillator 820. The first scintillator 810 andthe second scintillator 820 may be positioned adjacent one another suchthat at least a portion of light pulses generated within the firstscintillator 810 filters into the second scintillator 820 and reachesthe plurality of light-to-electrical converters 830, 831, 832, 833. Theplurality of light-to-electrical converters 830, 831, 832, 833 are eachoperably coupled to the second scintillator 820 in order to receive thelight pulses generated within the first scintillator 810 or the secondscintillator 820, and convert the light pulses into electrical signalsfor further processing, analysis, and data storage by an electronicsystem. The plurality of light-to-electrical converters 830, 831, 832,833 may be configured as one or more of a photomultiplier tube (PMT), aphotodiode, a charge-coupled device (CCD), a CMOS image sensor, or othersuitable device for converting received light-to-electrical signals.While four light-to-electrical converters 830, 831, 832, 833 are shownin FIG. 8, any number of light-to-electrical converters may be employeddepending on the surface area of the LABS 800.

The LABS 800 may further include a guard element (not shown) that may bestructured similar to the guard element 140 described above withreference to FIG. 1, for example, to filter certain low energy radiationfrom entering into the first scintillator 810, as well as suppressingother background effects from the soil. The additional descriptionreferring to FIG. 1 may also be applicable to the LABS 800 of FIG. 8,such as, for example, methods of bonding the various materials (e.g.,using optical coupling grease), preparing (e.g., polishing, coating,etc.) surfaces of the materials, as well as the selection ofscintillator materials used for the first scintillator 810 and thesecond scintillator 820, to exhibit different timing characteristics.Therefore, further description regarding the different materials andmanufacturing methods of the LABS 800 is not repeated with respect tothe embodiment of FIG. 8.

As discussed above, radiation (not shown) interacts with the LABS 800,which generates light pulses in the first scintillator 810 and thesecond scintillator 820. These light pulses are received by theplurality of light-to-electrical converters 830, 831, 832, 833 forconversion to electrons that may be received by electronic hardware andsoftware (see FIG. 4) for processing, analysis, and data storage. Thefirst and second scintillators 810, 820 exhibit at least one mutuallydifferent, distinguishing characteristic in the detector response suchthat the electronic hardware and software may determine which lightpulses were generated in the first scintillator 810 and which lightpulses were generated in the second scintillator 820, as well as theenergy of those light pulses. An example of such a distinguishingcharacteristic is the pulse shape characteristic (i.e., rise time) forthe light pulses respectively generated in the crystals of the first andsecond scintillators 810, 820. Because the electronics may also beconfigured to measure the rise time of the light pulses, the relativepulse contribution from each of first and second scintillators 810, 820may be determined Further, the light energy of the pulses generated inboth the first and second scintillators 810, 820 may also be measured,such as by combining the timing and energy data received from the firstand second scintillators 810, 820, as discussed above. As a result,discrimination of particular radionuclides of interest may be performedboth based on rise time and deposited energy. For example, the timingdata taken from each of the first and second scintillators 810, 820 maybe correlated with the energy spectrum obtained. Background effects fromprimary background radionuclides may be subtracted from the detectorresponse based on both the scintillator in which the energy deposits,and the background suppression from other scintillators that areexpected to capture the radiation from the particular radionuclide ofinterest.

The first and second scintillators 810, 820 may be structured (e.g.,customized) such that the receiving electronic hardware and software mayperform energy discrimination and background suppression on the lightresponse from each of the scintillators 810, 820. In other words, thethicknesses (T₁, T₂) of the first and second scintillators 810, 820 maybe dependent on the radionuclide desired to be measured. For example,the first scintillator 810 may be structured to permit substantially allof the energy deposition of a particular low-energy radionuclide. Inother words, the thickness (T₁) of the first scintillator 810 may beselected, for the particular material thereof, such that a majority ofthe energy from the low-energy radionuclide will interact within thematerial of the first scintillator 810 to ensure a goal of full energydeposition in the first scintillator 810 for energy from a particularradionuclide of interest. Because all, or substantially all, of theenergy of the particular low-energy radionuclide is deposited in thefirst scintillator 810, the emissions from that particular low-energyradionuclide may not reach into the second scintillator 820. The firstscintillator 810 may also structured to permit other radiation to bepassed therethrough to the second scintillator 820 in order to interactin the second scintillator 820.

The second scintillator 820 may have a thickness for detection ofhigher-energy radiation, such as high energy beta particles (e.g., >300keV) and gamma rays. The selected thickness (T₂) for the secondscintillator 820 may be based on ensuring a goal of full energydeposition of other expected radiation exhibiting higher energies.Selecting the thickness of second scintillator 820 for full energydeposition of high-energy radiation based on the material chosen forsecond scintillator 820 allows both rise time and energy discriminationto be more accurately performed and quantified, even among radionuclidesthat emit the same radiation type. For example, beta particles from oneradionuclide may be deposited in the first scintillator 810, while betaparticles from another radionuclide may be deposited in the secondscintillator 820. Examples of primarily beta-emitting or low-energygamma ray radionuclides that may be detected and discriminated from eachother include Tc-99, H-3, Sr-90, Y-90 C-14, and I-129. In addition,gamma ray emitting radionuclides that can be detected in the detectorand discriminated from the beta emitting radionuclides include Cs-137,Co-60, K-40, U-235, U-238, and Am-241.

The LABS 800 may be configured to have a detector area that isrelatively larger than the detector assemblies of FIGS. 1 through 3. TheLABS 800 may include large-area cast plastic scintillators. For example,the surface area of the LABS 800 may be in the square meter range. As aresult, the LABS 800 may be better suited for detection of radionuclidesover a large area, such as being used to scan a large land region for arange of low-energy beta and gamma-ray emitting radionuclides. The LABS800 may also be used to measure larger area water or air regions thatmay be contaminated with these radionuclides. In other words, the LABS800 may be a large area specialized multi-layer detector and analysissystem for use in scanning large areas to relatively rapidly quantifythe distribution of radionuclides that are difficult to detect usingconventional methods. The LABS 800 may be particularly useful in fieldenvironments, such as those that might be around nuclear reactors,nuclear waste processing sites, or medical facilities.

The first and second scintillators 810, 820 may be mounted on a metalstructural assembly to provide structural support for the LABS 800. Forexample, the LABS 800 may be mounted on a vehicle to drive over an areaand collect the radionuclide data. A technician may also walk the LABS800 around the area. The materials used, as well as the structure of theLABS 800, enables the LABS 800 to be relatively lightweight comparedwith conventional detectors that require heavy shielding materials fordiscrimination. The light weight may also provide easier transport ofthe LABS 800, as well as reduced costs. The structure of the LABS 800further enables detection and discrimination of lower energyradionuclides that may not be individually detected by conventionalmethods. In addition, a real-time response may be monitored in a varietyof different media (e.g., air, water, soil). For example, demonstrateddetection limits for some radionuclides have been as low as 1 pCi/g withmeasurement times of less than one minute.

The LABS 800 may be further associated with positioning equipment (e.g.,a global positioning system (GPS)) that associates the collected datawith a particular location, which may be used to recreate a map of thearea to show where the particular radionuclides were detected. In someembodiments, the LABS 800 may be mounted at a specific location, such asin a stream, at a critical checkpoint for weapons detection, etc.) tocontinuously monitor that location for changes in radionuclide movementacross that location. In another embodiment, the LABS 800 may be mountedto a remote controlled device (e.g., vehicle, robotic arm, etc.) thatmay permit radionuclide detection in locations where it is not desiredor possible for a technician to be present. In addition to these in situand field applications, the LABS 800 may be used in a laboratoryenvironment.

FIG. 9 is a schematic diagram of a large area radiation detector system(LABS) 900 according to another embodiment of the present disclosure.The LABS 900 may include a first scintillator 910, a second scintillator915, a third scintillator 920, and a plurality of light-to-electricalconverters 930, 931, 932, 933. Having another scintillator (three shownin this embodiment) enables the LABS 900 to further discriminate among aplurality of different radionuclides. For example, the firstscintillator 910 may be structured for full energy deposition of a firstlow-energy beta emitter (e.g., H-3), the second scintillator 915 may bestructured for full energy deposition of a second low-energy betaemitter (e.g., TC-99) (the second low-energy beta emitter has a higherenergy than the first low-energy beta emitter) and the thirdscintillator 920 may be structured for full energy deposition of ahigher energy radiation emitter (e.g., SR-90). Additional embodimentsmay include more scintillators structured for further discrimination ofadditional radionuclides. As a result, the LABS 900 may be tuned todetect and discriminate between a variety of radionuclides, includingalpha emitters, beta emitters, gamma emitters, and combinations thereof.

FIG. 10 is a schematic diagram of a large area radiation detector system1000 according to another embodiment of the present disclosure. The LABS1000 may include a plurality of first scintillators 1010, 1011, 1012,1013, a plurality of second scintillators 1020, 1021, 1022, 1023, and aplurality of light-to-electrical converters 1030, 1031, 1032, 1033.While the plurality of light-to-electrical converters of FIGS. 8 and 9are shown to receive light from continuous sections of scintillators,the plurality of light-to-electrical converters 1030, 1031, 1032, 1033may correspond to discrete sections of an array of scintillators thatcombine to form the LABS 1000. For example, a first light-to-electricalconverter 1030 may correspond to the first scintillator 1010 and thesecond scintillator 1020 to receive light therefrom. A secondlight-to-electrical converter 1031 may correspond to the firstscintillator 1011 and the second scintillator 1021 to receive lighttherefrom. A third light-to-electrical converter 1032 may correspond tothe first scintillator 1012 and the second scintillator 1022 to receivelight therefrom. A fourth light-to-electrical converter 1033 maycorrespond to the first scintillator 1013 and the second scintillator1023 to receive light therefrom. Each individual scintillator may have adifferent thickness, material composition, or both, for discriminationof different radionuclides. For example, each of the plurality of firstscintillators 1010, 1011, 1012, 1013 may have different thicknesses forfull energy deposition of emissions from a different radionuclide.Although not specifically shown, it is contemplated that the thickness,material composition, or both may be different for one or more of theplurality of second scintillators 1020, 1021, 1022, 1023.

As a result, the LABS 1000 may discriminate between a larger number ofdifferent radionuclides using vertical division having differentthicknesses rather than the additional horizontal sheets ofscintillators (as in FIG. 9). Of course, it is contemplated thatadditional scintillators may be present (e.g., each column of the arraymay include three or more scintillators of various thicknesses). Thedata analysis modules, such as data acquisition hardware (FIG. 4) andsoftware modules (FIG. 4) may receive each data set individually fromeach of the plurality of light-to-electrical converters 1030, 1031,1032, 1033, and may perform a combined analysis for measuring deviationsfrom the background, discriminating the different radionuclides, mappingthe data, etc.

CONCLUSION

An embodiment of the present disclosure includes an apparatus fordetecting a radiation source. The apparatus comprises a firstscintillator and a second scintillator adjacent to the firstscintillator. Each of the first scintillator and second scintillator maybe structured to generate a light pulse responsive to interacting withincident radiation. The first scintillator may be structured toexperience full energy deposition of a first low-energy radiation, andpermit a second higher-energy radiation to pass therethrough andinteract with the second scintillator. The apparatus further comprises aplurality of light-to-electrical converters operably coupled to thesecond scintillator and configured to convert light pulses generated bythe first scintillator and the second scintillator into electricalsignals. The first scintillator and the second scintillator exhibit atleast one mutually different characteristic for an electronic system todetermine whether a given light pulse is generated by the firstscintillator or the second scintillator.

Another embodiment of the present disclosure includes an apparatus fordetecting a radiation source. The apparatus comprises a first detectorand an adjacent, second detector, each structured to generate a lightpulse responsive to interacting with incident radiation. The firstdetector may be structured to experience full energy deposition of alow-energy radiation emission from a particular radionuclide and permita second higher-energy radiation emission to pass therethrough andexperience full energy deposition within the second detector. Theapparatus further comprises a plurality of light-to-electricalconverters operably coupled to the second scintillator for convertingthe light pulses generated by the first detector and the second detectorinto electrical signals.

Yet another embodiment of the present disclosure includes a method fordetecting a selected radioactive activity in an environment. The methodcomprises detecting a first set of low-energy radiation pulses within afirst scintillator through a plurality of light-to-electricalconverters, detecting a second set of higher-energy radiation pulseswithin a second scintillator through the light-to-electrical converters,wherein a detection surface area of the first and second scintillatorsis greater than a surface area of an individual light-to-electricalconverter, and subtracting a portion of the first set of low-energy betaparticle radiation pulses related to an estimated contribution of pulsesgenerated in the first scintillator by the second set of higher-energyradiation pulses generated within the second scintillator.

While embodiments of the present disclosure are susceptible to variousmodifications and implementation in alternative forms, specificembodiments have been shown by way of non-limiting example in thedrawings and have been described in detail herein. However, it should beunderstood that the disclosure is not intended to be limited to theparticular forms disclosed. Rather, the invention includes allmodifications, equivalents, and alternatives falling within the scope ofthe following appended claims and their legal equivalents.

What is claimed is:
 1. An apparatus for detecting a radiation source,the apparatus comprising: a first scintillator divided into discretesections, wherein at least two of the discrete sections are structuredwith a different thickness; a second scintillator adjacent to the firstscintillator; each of the first scintillator and second scintillatorbeing structured to generate a light pulse responsive to interactingwith incident radiation, the first scintillator being structured toexperience full energy deposition of a first low-energy radiation, andpermit a second higher-energy radiation to pass therethrough andinteract with the second scintillator; and a plurality oflight-to-electrical converters operably coupled to the secondscintillator and configured to convert light pulses generated by thefirst scintillator and the second scintillator into electrical signals,wherein the first scintillator and the second scintillator exhibit atleast one mutually different characteristic for an electronic system todetermine whether a given light pulse is generated by the firstscintillator or the second scintillator.
 2. The apparatus of claim 1,wherein the at least one mutually different characteristic includes adifferent pulse shape of light pulses generated by the firstscintillator and the second scintillator.
 3. The apparatus of claim 1,wherein at least one of the first scintillator and the secondscintillator includes a plastic scintillator element.
 4. The apparatusof claim 1, wherein the electronic system is configured to receive thelight pulses and discriminate between a plurality of differentradionuclides that emitted the incident radiation.
 5. The apparatus ofclaim 4, wherein the plurality of different radionuclides includes atleast two radionuclides selected from the group consisting of Tc-99,H-3, Sr-90, Y-90, Cs-137, Co-60, Th-232, K-40, U-235, U-238, C-14,I-129, Ni-63, and Am-241.
 6. The apparatus of claim 1, wherein the firstlow-energy radiation and the second higher-energy radiation is of a sameradiation type.
 7. The apparatus of claim 6, wherein the same radiationtype includes beta particles.
 8. The apparatus of claim 1, wherein eachof the first scintillator and the second scintillator are continuouslayers shared by the plurality of light-to-electrical converters.
 9. Theapparatus of claim 1, further comprising a guard element adjacent to thefirst scintillator, wherein the guard element is configured to protect asurface of the first scintillator.
 10. The apparatus of claim 9, whereinthe guard element is further configured to prevent entry of alphaparticles into the first scintillator.
 11. The apparatus of claim 1,wherein the second scintillator is structured to experience full energydeposition of the second higher-energy radiation.
 12. An apparatus fordetecting a radiation source, the apparatus comprising: a firstscintillator and an adjacent, second scintillator, each structured togenerate a light pulse responsive to interacting with incidentradiation, wherein the first scintillator is structured to experiencefull energy deposition of beta particles from a first particularradionuclide of interest and permit a second higher-energy radiationemission to pass therethrough and experience full energy depositionwithin the second scintillator; and a plurality of light-to-electricalconverters operably coupled to the second scintillator for convertingthe light pulses generated by the first scintillator and the secondscintillator into electrical signals.
 13. The apparatus of claim 12,further comprising a third scintillator adjacent to the secondscintillator, wherein the third scintillator is structured to generate alight pulse responsive to interaction with incident radiation, whereinthe third scintillator is structured to experience full energydeposition of a third higher-energy radiation emission after passingthrough the first scintillator and the second scintillator.
 14. Theapparatus of claim 12, wherein the second scintillator is structured toexperience full energy deposition of beta particles from a secondparticular radionuclide of interest.
 15. The apparatus of claim 12,further comprising: data acquisition hardware configured to determinewhether a given light pulse is generated in the first scintillator or inthe second scintillator based on a mutually different characteristicexhibited by the first scintillator and the second scintillator; and atleast one software module configured to separate out counts from thefirst low-energy radiation emission from counts attributable to thesecond higher-energy radiation emission in the first scintillator. 16.The apparatus of claim 15, wherein the mutually different characteristicincludes a rise time for the light pulses generated in the firstscintillator and the second scintillator.
 17. The apparatus of claim 15,wherein the at least one software module is configured to discriminatebetween radiation emissions from a plurality of different radionuclidesthat emit a same radiation type.
 18. The apparatus of claim 17, whereinthe at least one software module is configured to discriminate betweenradiation emissions from a plurality of different radionuclides thatemit a different radiation type.
 19. The apparatus of claim 15, furthercomprising a mounting structure having the first scintillator, thesecond scintillator, and the plurality of light-to-electrical convertersmounted thereon.
 20. The apparatus of claim 19, wherein the mountingstructure is configured to scan the first scintillator and the secondscintillator over an environment.
 21. The apparatus of claim 19, whereinthe mounting structure is at a fixed location.
 22. The apparatus ofclaim 19, further comprising a global positioning system operablycoupled with the data acquisition hardware for providing locationalinformation associated with the light pulses generated from the incidentradiation.
 23. The apparatus of claim 22, wherein the at least onesoftware module is further configured to create a map of an environmentby locating positions where incident radiation was detected with anidentification of particular radionuclides discriminated by the at leastone software module.
 24. The apparatus of claim 12, wherein the firstscintillator and the second scintillator are divided up into verticalcolumns corresponding to individual light-to-electrical converters ofthe plurality of light-to-electrical converters.
 25. The apparatus ofclaim 24, wherein at least two of the vertical columns differ in atleast one of a thickness and a material composition for the firstscintillator.
 26. A method for detecting a selected radioactive activityin an environment, the method comprising: detecting a first set oflow-energy radiation pulses within a first scintillator through aplurality of light-to-electrical converters; detecting a second set ofhigher-energy radiation pulses within a second scintillator through thelight-to-electrical converters, wherein a detection surface area of thefirst and second scintillators is greater than a surface area of anindividual light-to-electrical converter; and subtracting a portion ofthe first set of low-energy beta particle radiation pulses related to anestimated contribution of pulses generated in the first scintillator bythe second set of higher-energy radiation pulses generated within thesecond scintillator.
 27. The method of claim 26, wherein detecting thefirst set of low-energy radiation pulses includes obtaining pulses froma full energy deposition in the first scintillator for a particularlow-energy beta particle emitter of interest.
 28. The method of claim27, wherein detecting the second set of higher-energy radiation pulsesincludes obtaining pulses from a full energy deposition in the secondscintillator for a specific higher-energy beta particle emitter.